1. Field of the Invention
This invention relates to a linear driving device obtained by combining a hydrostatic gas bearing or a direct-acting bearing and a voice-coil motor.
2. Description of the Prior Art
FIG. 9 shows a linear driving device used in a conventional precision measuring apparatus or the like. A voice-coil motor, comprising a coil assembly 103a and a magnetic-circuit assembly 103, is disposed at the side of one end of a shaft 102 having a rectangular cross section supported by a hydrostatic gas bearing 101. A linear encoder 104 is mounted at the side of the other end of the shaft 102, and a measuring apparatus or the like is mounted at an end portion 102a of the shaft 102. The voice-coil motor moves the shaft 102 to a target position in accordance with a positional signal from the linear encoder 104.
On the other hand, in Japanese Utility Model Application Public Disclosure (Kokai) No. 59-72317 (1984), there is disclosed an approach such that in a shaft and a bearing having circular cross sections, in order to restrain the degree of freedom of the rotation of the shaft, an engaging unit for preventing rotation having a fluid outlet is provided between the shaft and the bearing.
In the configuration shown FIG. 9, however, since the voice-coil motor and the hydrostatic gas bearing are linearly arranged, the size of the device becomes large. Furthermore, in order to configure a hydrostatic gas bearing having a rectangular cross section, a large number of components are needed, and it is necessary to perform adjustment operations, such as gap adjustment and the like, in assembling the components, causing an increase in the production cost. In the configuration of the above-described publication, since the engaging unit for preventing rotation is provided between the shaft and the bearing, and the engaging unit and a unit engaged therewith form an air outflow channel provided in the inner wall of a long hole or the inner wall of the bearing, it is difficult, for example, to process these units with high precision and adjust gaps between them.
Furthermore, in the above-described conventional approaches, if an external force or the like is applied to the device when the supply of pressurized gas is stopped due to malfunctioning of the apparatus, human error, or the like, and the shaft contacts the bearing, the shaft moves when it is in a contact state with the bearing, causing damage to the shaft and the bearing.
Moreover, the gap between the coil and the yoke of the motor is the only portion of the device communicating with the outside (air) in the closed space surrounded by the coil of the voice-coil motor, and therefore the efficiency of exhaust is poor. Hence, the coil and the shaft tend to be displaced (in the order of 0.1 .mu.m) in the axial direction (the direction of a positioning operation) due to a change in pressure accompanying a change in the volume of the closed space during a driving operation for positioning, causing vibration in the axial direction, and causing a deterioration in the accuracy of a positioning operation and lengthening the time period of the positioning operation.